The present invention relates to a caulking material used for filling and sealing interstices in various structured bodies to prevent leakage of water therethrough. More particularly, the invention relates to a water-swellable caulking material having a stratified structure of a composite body formed of a plural number of integrally bonded layers comprising (A) at least one layer of a water-swellable rubbery composition containing particles of a highly water-absorbing resin dispersed in a rubbery matrix and (B) at least one layer of a rubbery material not swellable with water which serves to resist or suppress the expansion of the composite body within the plane of the layers when the water-swellable layer (A) is swollen by absorbing water so as to limit the swelling expansion of the composite body as a whole only in the direction of the thickness of the body whereby leakage of water through the interstice of a structured body can be completely prevented irrespective of the running direction of water.
There are known several rubbery compositions compounded with particles of a water-absorbing polymeric material, such as a polyvinyl alcohol, polymer of an acrylic acid salt, carboxymethylcellulose, hydroxyethylcellulose and the like, dispersed in the matrix of a rubber (see, for example, Japanese Pat. Kokai No. 53-143653, 54-7461, 54-7463 and 54-20066) which are capable of expanding when swollen with water and useful as a caulking material for preventing water leakage. These compositions have several problems as a caulking material that the swelling pressure as one of the most important characteristics for the caulking effect cannot be sufficiently high because of the relatively low swelling ratio of the material due to the solubility of the dispersed phase of the water-absorbing material in water and that the velocity of water absorption is low so that no instant caulking effect can be obtained with the composition. Further, there is also known a rubbery composition in which the water-absorbing dispersed material is a urethane polymer. The swelling pressure obtained with a caulking material of this type is also relatively low due to the insufficient affinity of the urethane polymer to water in addition to the problem of the extremely low velocity of water absorption.
Recently, there is disclosed a water-swellable composition (see, for example, Japanese Patent Kokai 54-94525 and 54-110262) which is a combination of a copolymer of an .alpha.-olefin and maleic anhydride as the water-swellable but water-insoluble polymeric component and a nitrile-based coating material or natural rubber as the non-swellable component. The compositions of this type are useful in their own manner when used as a water-swellable coating composition or as a caulking material even though sufficiently high swelling pressure can hardly be obtained with them. That is, the rubbers such as nitrile rubbers and natural rubbers which are amorphous at room temperature may exhibit cold flow in their unvulcanized state so that the expansion of the highly water-absorbing resin by swelling with water is absorbed by the cold flow of the rubbery component not to produce sufficiently high swelling pressure at the place where a high swelling pressure is essential in order to obtain satisfactory caulking effect. A remedy for this shortcoming may be provided when the rubber component is vulcanized but the crosslinked network structure of the rubber molecules is sometimes a barrier against the expansion of the highly water-absorbing resin by swelling so that no satisfactory swelling pressure can be obtained even by this means of vulcanization.
Alternatively, two of the present co-inventors have previously conducted investigations to solve the above problems and proposed efficient caulking materials in the form of a sheet or string shaped of a rubbery composition comprising a 1,3-diene-based rubber, of which the content of the crystalline or glassy region is 5 to 50% at room temperature, and a specific highly water-absorbing resin dispersed in the matrix of the rubber. The caulking material of this type is indeed effective for preventing water leakage through interstices in structured bodies though still having problems that the caulking material expands when swollen with water in all directions not only perpendicularly to but also within the plane of the applied surface so that the swollen caulking material sometimes gets out of the interstice of the structured body resulting in the loss of the desired caulking effect.